Many of us are all too familiar with having cut our finger on the sharp edge of paper or a paper product, such as cardboard, corrugated paperboard, a sheet of writing paper, etc. For example, as one might run one's finger underneath a sealed envelope to open it, the sharp edges on the flap, as it is being torn open, sometimes tears into the finger, causing a painful, sometimes bleeding cut that is prone to infection. Some grades of paper are so sharp along the edge, that many persons accidentally cut themselves when routinely handling the paper in normal use. This is often the case, with containers and packaging, when persons pack and seal, open and unpack paper-based materials.
There are other materials, similar to paper, where cuts occur when persons contact the material. While what follows applies primarily to paper products, the invention is suitable with other similar material.
The outer layer of human skin is made up of dead cells that are immersed in a matrix of fibrous secretions. A "cut" occurs when the outer layer is torn or slit and the structures below, live cells, lymph, blood vessels, etc., are reached. A cut can occur when an edge of a paper product contacts skin, if the edge is thin enough or sharp enough, relative to skin structure, and if the material behind the edge is stiff enough so sufficient stress can be developed.
These conditions follow from some common observations: while tissue paper is thin, it does not provide enough support for its edge to cut skin; a sheet of writing paper, which might otherwise be able to cause a cut, is unlikely to do so when the paper is crumpled and its edge lacks regularity and stiffness; a cut is more likely to occur when the edge of a sheet of paper slides across, rather than when the same sheet only presses up against the skin; soft paper, particularly paper having short fiber lengths, usually does not have sufficient structural rigidity to tear through skin; stiffer papers with long fiber contents have greater rigidity and are more apt to penetrate the skin.
Two terms commonly used to describe paper are (basis) weight and stiffness. The weight of a paper is its weight in pounds per 1000 square feet (of face area). The specific weight of a sample of the paper would be given as the ratio of basis weight to its thickness. Typical weights are listed below:
______________________________________ type weight, lbs in .times. in - sheets/ream ______________________________________ mimeograph paper 21 17 .times. 22 - 400 newsprint 32 24 .times. 36 - 500 machine finish book 48 17 .times. 22 - 500 supercalendered book 50 25 .times. 38 - 500 coated book 70 25 .times. 38 - 500 ______________________________________
The stiffness of paper is directly proportional to the product of Young's modulus (E) and the cross sectional moment of inertia (I), and is inversely proportional to basis weight. Paper is a plastic material and E must be defined within a limited range of applied stress and taking the water content of the sample into account.
Corrugated paperboard products are made up of a plurality of layers of paper. A person handling an edge of any of these layers may be cut. Standard corrugated paperboard consists of various combinations of flat sheets and serpentine (fluted) sheets, of various height and number of flutes/foot, glued together in alternating layers. Corrugated materials in common use are described as follows:
______________________________________ number of flat number of type sheets serpentine sheets ______________________________________ single face 1 1 double face 2 1 double wall 3 2 triple wall 4 3 ______________________________________
The sizes of double face corrugated boards that are in common use are as follows:
______________________________________ size flutes/foot height without faces take-up factor* ______________________________________ A 36 3/16" 1.58 B 51 3/32" 1.38 C 39-42 9/64" 1.5 E 96 3/64" 1.3 ______________________________________ *take-up factor is the ratio of the length of paper used in the fluted layer to the given length of board
Two general means by which paper and paper products are cut:
1) by drawing a sharp blade across and through the paper, and, PA1 2) by pressing a sharp die against the paper until penetration results in separation.
In either of these cases, the edge of the cutting tool may be straight or serrated (wave). Die-cutting is a considerably more expensive means that is used to obtain parts of complex shape, as well as to obtain dimensional accuracy. One of the things accomplished when paper products are cut with a wave die is that the user is less apt to receive a skin cut when handling the product. Serrated edges are irregular and increase the amount of material which skin might contact, thereby reducing stress. While this method of cutting paper reduces the problem addressed by the present invention, people still receive cuts from the serrated edge of die-cut paper, and die-cutting is not practical for many products where cuts frequently occur.
Additional information about paper can be obtained from the AMERICAN PAPER INSTITUTE, 1250 Connecticut Avenue, N.W., Suite 210, Washington, D.C. 20036. A standard reference is the HANDBOOK OF PULP AND PAPER TECHNOLOGY, edited by Kenneth W. Britt, VAN NOSTRAND REINHOLD, 1970.